The expression of transforming growth factor beta 1 (TGF-beta 1) mRNA has been determined in 16 breast carcinomas using in situ hybridization and compared with TGF-beta protein as detected by antibodies against TGF-beta 1 and TGF-beta 1 plus TGF-beta 2. Digoxigenin-labelled riboprobes were used, with alkaline phosphatase and immunogold silver detection systems. TGF-beta 1 mRNA was only detected in carcinomas in which TGF-beta 1 protein was found (9 of 16 cases) and not in those with prominent reactivity for TGF-beta 2. RNA preservation was poor in two other cases in which TGF-beta 1 protein had been detected. In general, those tumours with greater numbers of cells labelled for TGF-beta 1 mRNA had prominent reactivity for TGF-beta 1 protein. The mRNA was localized to cancer cells with no labelling of stromal cells, although in a small number of cases scanty staining for TGF-beta 1 protein had been observed in stromal cells. The incidence of detection of TGF-beta 1 mRNA is lower than the published data from Northern analysis studies of breast carcinomas, suggesting that only higher levels of TGF-beta 1 mRNA expression are being detected by in situ hybridization. However, this approach has provided useful information about the cellular sites of expression of TGF-beta 1 in breast carcinomas.